1. Field of the Invention
The present invention relates to an olefin polymerization catalyst comprising a metallocene compound, an organoaluminum compound, and a metal borate or aluminate, and a process for polymerizing olefin by use of the catalyst.
2. Description of the Related Art
Various catalysts are known for olefin polymerization. Japanese Patent Application Laid-Open No. 3-197513 discloses polymerization of ethylene by use of a metallocene compound and an organoaluminum compound as the catalyst. Japanese Patent Application Laid-Open No. 3-290408 discloses a process for producing polyethylene or ethylene copolymer by use of a zirconocene compound, an organoaluminum compound, and an organomagnesium compound as the catalyst. These catalysts, however, are not satisfactory in activity in polymerization of propylene although they are useful for production of polyethylene and ethylene copolymers.
Further, Japanese Patent Application Laid-Open No. 58-19309 and other papers disclose processes of polymerization of olefin including propylene with a catalyst constituted of a metallocene compound and methylaluminoxane with high activity. This catalyst system has disadvantages of high cost of the methylaluminoxane and residual aluminum in the formed polymer a relatively large amount of the methylaluminoxane needs to be used.
Japanese Patent Application Laid-Open No. 3-207704 discloses a highly active catalyst for polymerization of olefin including propylene: the catalyst composed of an ionic metallocene is activated by addition of an organoaluminum compound. The ionic metallocene, the main catalyst, is generally prepared by reaction of a metallocene compound with an ionizing ionic compound. The ionizing ionic compound for preparing the ionic metallocene catalyst is synthesized by converting a metal borate or a metal aluminate into an ammonium salt or a triphenylcarbenium salt. Therefore, this catalyst system has disadvantages that synthesis of the ionizing ionic compound requires many steps of operations and involves many problems on the purity thereof, reproducibility in the synthesis, deactivation during storage and on transfer to a polymerization vessel, and so forth.
The inventors of the present invention made comprehensive investigation to solve the above problems, and found that olefin is polymerized with a catalyst formed from a metallocene compound, an organoaluminum compound, and a metal borate or aluminate, which is to be converted to the ionizing ionic compound, with high catalyst activity to produce polyolefin of high moldability.